The present invention refers to a heat exchanger plate for a plate package for a plate heat exchanger. The invention also refers to a pair of heat exchanger plates comprising a first heat exchanger plate and a second heat exchanger plate. Furthermore, the invention refers to a plate package for a plate heat exchanger.
Such plate heat exchangers with heat exchanger plates of the initially defined kind comprise commonly a central heat transfer area with a so-called herringbone pattern, which means that the corrugations of a parallel ridges and valleys of adjacent plates abut each other in such a way that substantially point-shaped contact surfaces are formed between the plates, and with distribution areas at the portholes on the end areas of the heat exchanger plates. The distribution areas are commonly provided with so-called distribution patterns (“chocolate patterns”), i.e. the corrugations of adjacent plates are designed in such a way that they form substantially line shaped contact surfaces between adjacent plates. In the transition between the distribution area and the central heat transfer area, i.e. where the corrugation patterns change design, the strength becomes somewhat reduced than at the central heat transfer area proper and the distribution areas proper. The reason therefor is that the different construction methods of the corrugation patterns, which means that the herringbone pattern has many small closely positioned contact surfaces whereas the distribution pattern has large but few contact surfaces with a free structure therebetween.
FIG. 1 discloses schematically a heat exchanger plate 1 where, in an area at the top to the right, a pattern has been drawn in the transition between the central heat transfer area 2 and the distribution area 3. In FIG. 2 this area is shown in a larger scale. The rhomboids 4 correspond to the line-shaped contact surfaces at the bottom plane of the heat exchanger plate 1 and the rhomboids correspond to the line-shaped contact surfaces at the upper plane of the heat exchanger plate 1. The lines 6 are valleys of the heat exchanger plate 1 concerned whereas the lines 7 are the ridges of an adjacent heat exchanger plate 1. Where the valleys 6 cross the ridges 7 point-shaped contact surfaces are created, which absorbs pressure load. In FIG. 2, the herringbone pattern of the central heat transfer area 2 is a typical so called pattern with high NTU (Number of Heat Transfer Units) with an acute angle of approximately 65° between the ridges and a centre axis x in the longitudinal direction of the heat exchanger plate 1. FIG. 3 discloses a typical so called pattern with low NTU with a corresponding acute angle of approximately 25°. The pattern with high NTU gives a relatively high flow resistance whereas the pattern with low NTU gives a relatively low flow resistance.
The pattern with high NTU gives the distance A1 along the width between the contact surfaces, which is significantly larger than the corresponding distance A2 of the pattern with low NTU. In the transition to the distribution area, this is of great importance for the strength since the contact surfaces have to take a part of the load on the distribution area. If the distance A1 is compared with A2 it can be seen that A1 is twice as long as A2. Since the number of contact surfaces in the row is reciprocally proportional to the distance, the pattern with low NTU will give twice as many support points as the pattern with high NTU along the transition to the distribution area. The longer the distance along the width between the contact surfaces is, the larger the load on each contact surface will be, and it is difficult to avoid large free surfaces which are highly loaded. In addition to a higher load on the contact surfaces on the pattern with high NTU, a collapsing load for the fields in the distribution area also becomes lower.
The heat exchanger plate with pattern with high NTU on the central heat transfer area will thus determine the maximal pressure performance for the heat exchanger plates in the cases when this area is dimensioning. If the heat exchanger plates always are provided with a pattern with low NTU on the central heat transfer surface, the above mentioned strength problems will not occur. However, in many cases it is desirable to use a so-called pattern with high NTU on the central heat transfer area in order to obtain a high heat transfer.
U.S. Pat. No. 4,781,248 discloses a heat exchanger plate of the initially defined type. This heat exchanger plate is intended to be included in a plate package for a plate heat exchanger. It is especially referred to FIG. 4 in this document, which discloses a distribution area with a distribution pattern and a central heat transfer area with a pattern with high NTU.